The present embodiments relate to an arrangement for converting an analog detection signal from a magnetic resonance detection coil into a digital detection signal.
In magnetic resonance devices, local coils may also be used as detection coils. Such local coils transfer the detection signals to an evaluating device of the magnetic resonance device, where the signals are to be further processed by digital signal processing (DSP). Since the analog detection signals of the detection coil lie at the magnetic resonance frequency and are to be transported undistorted through the magnetic fields of the magnetic resonance device and through components of the magnetic resonance device without causing interference, coaxial cables with sheath current filters are used in order to transfer the detection signals to the evaluating device, where the signals are digitized and further processed.
WO 2006/048816 A1 discloses that the conversion of analog detection signals into digital detection signals may be carried out directly at the detection coil. For this purpose, an analog-to-digital converter is used. The analog-to-digital converter is arranged spatially within a detection region of the detection coil. In this way, bulky and costly coaxial cables are dispensed with. Active detection elements may be selected in the digital domain, so that analog switching electronics may advantageously be dispensed with. This also simplifies the use of the local coils.
One problem that arises with the corresponding embodiment of a conversion and transmission arrangement of this type is the digital signal transmission from the detection coil to a digital receiver of the evaluating device. The overall data rate used is proportional to the product of the signal bandwidth, the channel count and the logarithm of the signal dynamic range. These properties are limited by the available data rate of the transmission technology used.
The frequency bandwidth of the magnetic resonance signal is determined by the maximum gradient and object size. The bandwidth may reach an order of magnitude of approximately 1 MHz. In theory, it would thus be sufficient to use a sampling rate of 2 MS/s. However, in order to simplify the requirements placed on the prefilter for preventing noise-related aliasing, a sampling rate of, for example, 10 MS/s is used at the analog-to-digital converter.
In order to achieve the best possible signal to noise ratio (SNR) and the fastest measuring speed, the magnetic resonance signal may be detected with an array of a plurality of individual detection antennae of a detection coil, and processed simultaneously. For example, 128 active channels may be operated in parallel.
The signal dynamic range to be processed at an input of the signal chain (e.g., at the detection coil) extends from the thermal noise floor from the object being detected (e.g., a patient (−174 dBm/Hz)) up to a maximum level of approximately −20 dBm in a magnetic resonance device having a basic field strength of 3 Tesla. So that rounding errors in the digital transmission of the background noise does not increase significantly, at a sampling rate of, for example, 10 MS/s, a word size of approximately 18 bits is used.
This results, in this example, in a total data rate of 18*128*10 Mbit/s=23 Gbit/s. This represents an obstacle for the realization of wireless (e.g., radio) data transmission between the detection coil and the evaluating device.
Ways to reduce the transmission rate are being sought. A sampling rate of 2 MS/s is theoretically sufficient to be able to image the 1 MHz bandwidth magnetic resonance signal (e.g., the useful signal) without loss. German application DE 10 2011 006578.4 proposed that the digitized detection signal should initially be decimated to the lowest possible data rate. This is undertaken only after conversion into a digital detection signal by the analog-to-digital converter, since the useful dynamic range of the analog-to-digital converter in logarithmic representation behaves proportionally to the sampling rate. Thus, for example, at a sampling rate of 5 MS/s, the available dynamic range is three decibels lower than at 10 MS/s. In addition to this, given a severe reduction in the sampling rate of the analog-to-digital converter (ADC); a halved rate of 5 MS/s), the demands made on the analog filter may be met with extreme difficulty or not at all. For example, the useful signal from the detected signal downmixed to an intermediate frequency lies at 1.8+/−0.5 MHz. In that case, the “roll-off” (e.g., the step in the frequency response between the pass band and the stop band; at −40 dB) is achieved at between 2.3 MHz and 2.7 MHz, where the first aliasing band would occur. This requirement may not be achievable, with the result that decimation of the digitized detection signal to a lower sampling rate by a decimation filter is proposed. However, the requirements exist. For example, with the roll-off at 2.3 MHz-2.7 MHz, the filter portion of the digital decimation filter suppresses alias bands and noise aliasing. These requirements increase strongly as the degree of decimation rises. Therefore, an ever more complex filter with an increasing number of adders, subtractors (e.g., in cascaded integrator-comb filters (CIC)) and multipliers (e.g., in the general form of finite impulse response (FIR) filters) is provided. Taking account of the available FGPA resources and, above all, the energy utilization, the limits of feasibility are quickly reached. Added hereto is that, with regard to the emission of interference signals, the smallest possible number of digital operations close to the magnetic resonance detection antennae of the detection coil is a goal.